Moreover, systemd has a OnFailure directive that activates other units when the process exits uncleanly. Upstart has a similar directive. With a more classical approach, `respawn' is a utility that can invoke and maintain the running status of a process launched from System V Init, or you could use the regarded inittab solution.
I've seen answers to questions similar to this on StackOverflow respond with scripts that are nothing more than a while(true) loop that checks to see if a PID exists in languages ranging from Bourne shell to Python and NodeJS. This is the wrong way to do it, especially the Node one. Lets take a language intended to be used inside a web browser client and slap it onto a server to be used for process monitoring. I'm sure it does a fine job with it's while loop, but we're hitting copper pipe with a tire iron wondering why our lightbulb isn't turning on here.